Audio Engineering - Sound Reinforcment

Ray By Ray - November 11th, 2017
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If a tree falls in the forest, and there is no one to witness, will it still make a sound? They should have asked a soundman ! Yes, the falling tree will move a lot of air, and the movement of air molecules translates into the sound we all here.
For the last 40 years I have made my living manipulating those sound particles. How? as an Audio Engineer for touring National and International bands and groups. You see I am the guy that sits out in the audience behind racks and racks of electronics and an audio mixing board. I mix the bands music for the audience to hear at a concert.
One might think that when using a sound reinforcement system ( PA ) that the sound that you sang into the microphone is what you would hear out of the speakers, but no. The second the sound waves come out of the speakers they are effected by environment in which they stand.
It's the Audio Engineers (Soundman) job to be able to recognize these changes and thru the use of electronics compensate for them, to give the listening audience the truest representation of " What goes in, Is What comes out."
I will be discussing here the principles of sound reinforcement.
Now back to science class we see that our ear is made up of many pieces. As the sound waves enter our ears, the first encounter a thin dome called Tympanic Membrane  that is connected to tiny bones within the ear. As the air molecules from the outside world come pouring in, our Tympanic Membrane vibrates at the same rate or frequency. This vibration then excites the inner ear bones that in turn vibrate, theses bones then pass the signal on to nerves that are sent to the brain. The brain then processes the information and we hear. Humans perceive frequency of sound waves as pitch  so the more sound waves per second, the higher the pitch.

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The perfect example of this is by dropping a stone into water. Think of sound as air molecules moving thru the air, like the ripples of the water. The frequency of the ripples, in the sound world, we measure in cycles per second and are represented by the term Hertz. (Hz) Although the human ear is able to perceive frequencies ranging from 10 Hz to 20,000 Hz, the average human can only hear sounds between 16 Hz and 20,000 Hz. The lower the sound in pitch the lower the frequency is and therefore a lower number or Hertz.

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Another visualization would be a guitar string being plucked.

Each frequency we hear has what we will call peaks and troughs. Think back to the rock and water, the frequency of the note is determined by how many lets say peaks measured over 1 second. As the guitar string is plucked, the string moves air molecules in two directions creating the peaks and troughs we see in the diagram.

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These sound waves contain important information. The amplitude; or how loud it is, the frequency; or the pitch of the sound and the wavelength; how much distance is needed for the wave to complete one cycle.

Amplitude or loudness is measured in decibels, the frequency is measured in Hertz and the wavelenght is measured in feet.

 

The decibel (abbreviated dB) is the unit used to measure the intensity of a sound. The decibel scale is a little odd because the human ear is incredibly sensitive. Your ears can hear everything from your fingertip brushing lightly over your skin to a loud jet engine. In terms of power, the sound of the jet engine is about 1,000,000,000,000 times more powerful than the smallest audible sound. That's a big difference!

On the decibel scale, the smallest audible sound (near total silence) is 0 dB. A sound 10 times more powerful is 10 dB. A sound 100 times more powerful than near total silence is 20 dB. A sound 1,000 times more powerful than near total silence is 30 dB. Here are some common sounds and their decibel ratings:

Near total silence - 0 dBA whisper - 15 dBNormal conversation - 60 dBA lawnmower - 90 dBA car horn - 110 dBA rock concert or a jet engine - 120 dBA gunshot or firecracker - 140 dB

You know from your own experience that distance affects the intensity of sound -- if you are far away, the power is greatly diminished. All of the ratings above are taken while standing near the sound.

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The above graphic is of a Sound Preasure Level (SPL) meter. This meter reads the preasure of the sound waves in their respect to air molucule movement.

Sound pressure level (SPL) or sound level is algorithmic measure of the effective sound pressure of a sound relative to a reference value. It is measured in decibels(dB) above a standard reference level. The standard reference sound pressure in air  is 20 uPa, which is usually considered the threshold of human hearing (at 1 KHZ).

The device consists of three sections:

The input microphone and it's level selecting dialThe Speed and Weight ControlsThe display meter.

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Above we can see the input microphone on the top of the unit and the Level selecting dial beneath. The dial adjusts the device to read levels from 60db to 120db

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The above graphic shows the "Weighting and Response" controls.The A and C settings are in relationship to overall frequency, -A- is normal and -C- hears more "Low Frequency" sounds. The Fast or Slow settings on the "Response" switch, determines how quickly the unit will react to changing audio levels.

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The graphic above shows the units db meter. To operate the device you first turn it on and select you base level from the turn dial. Select your "weighting and response times" and view the level on the units meter.

For example: If you set the level dial at its maximum of 120db then any signal that the device hears that reacechs 120db will read  0 (zero) on the scale. If the meter reads +4 than the total DB level is 124db. If the scale read -10 than the total db would be 110db.


Frequency and loudness are separate from each other. Two sounds may have the same frequency and different amplitudes, and vice versa.

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Wavelength is the converse of frequency: the shorter the wavelength, the higher the frequency; the longer the wavelength, the lower the frequency. This chart represent certain frequencies measured in Hertz and their respective wavelengths measured in feet. Notice that the lower the frequency or note, the longer the wavelength.

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One will notice that when we are standing on the curb waiting for the next rose parade band to come by, we will hear the low notes of the drums before we hear the highs of the piccolo or flute.

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Here we see a visual representation of one cycle of a sound wave. Each wave is made up of a starting point, a positive peak and a negative peak

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Phasing is a phenomenon that happens to sound when two identical notes or frequencies reach the listeners ears at slightly different times, this causes a noticeable loss in volume at that frequency. To understand Phasing, one must visualize the sound leaving the speakers are actually waves of air. As we see above this graph represents 1 cycle of the wave. It actually shows 2 identical waves where one wave “Blue” is slightly ahead of Red wave”.”The Blue wave” is 45 degrees out of Phase with the red wave..

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Starting at the top left we see on the first row a typical sound wave. On the next row down we see two sound waves of the same amplitude and frequency. If we skip over to the right side of the chart and look at the second row down from the top we see are same two sound waves, but they are 180 out of phase with each other. And as you can see from the graphic on the bottom right the two out of phase signals cancel each other out.

Now this comes all into play when you are placing microphones on stage. You see, if 2 mikes sample the same sound, phasing can take place. Here is the example; you have two conga drums that you are miking individually, we will give each drum a number, if drum 1 is struck it’s microphone will pick up the sound first, however the microphone on drum 2 is so close it also picks up the sound from drum 1  a few milliseconds later. The effect of phasing is this; the two waves CANCEL each other out! Producing a noticeable loss is signal and low end frequencies on either mike. Phasing also comes into play in the connection of speakers. It’s very important the (+) positive (red) connection on the speaker outs of the amplifier are connected to the (+) positive connection on the speaker. When a speaker receives a (+). Signal the cone will move in one direction and when it receives a (–) pulse it moves in the opposite direction. This movement is what recreates the sound wave we hear. Improper speaker connections cause one speaker to move OUT while the other speaker wants to move IN.  Causing the sound produced by the two speakers to be out of phase with each other.

 

A simple way to test to see if your speakers are in phase is to hold the end of a burning cigarette in front of the speaker. Turn on some tunes thru the system and watch to see the smoke from the cigarette blows out or in at the same time on all speakers. Newer mixing boards have a PHASE switch on each channel allowing you to change the phase of an input.

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… and yes woman.  From here on we need to look at sound as frequencies of air waves. From the high frequency screech of the bad brakes on our car to the low rumble of the garbage truck rolling by, we must begin to associate the sounds we hear into  their audio frequencies. From this point on we are not working with sounds per say as we are working with frequencies.

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Here we see a chart  created by our friends over at Independent Recording  www.independentrecording.net showing the frequency range of many instruments including the male and female voice.  The chart is divided into groups that represent the classes of instruments such as percussion, woodwinds, brass, strings and vocals. Across the top we can see the frequency range from 20 HZ to 20,000 HZ.

 

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This is a continuation of the chart. Notice the 5 different colored columns they represent the typical controllable ranges on most mixing boards. These are the sub bass, the bass, midrange, high midrange and highs. From this diagram you can also see the corresponding sections of a 1/3 octave graphic equalizer. Moving up the chart from the bottom you then see terms that are commonly used by engineers and musicians to describe specific areas of the overall sound.

 

Above that you see the layout of a standard 88 key piano and where frequencies fall into relationship with the keys.Take a few minutes and check out the interactive version of these charts. Study them well and save the link on your computer.  at http://www.independentrecording.net/irn/resources/freqchart/main_display.htm

Frequencies of speech:

We all know that our language is made up of vowels and consonants, in the soundmans world there is one more component called the fundamental. The fundamental, frequencies lie in a range between 100 and 250 hertz. These frequencies help us pick out who is speaking in a crowd. Vowels make up the power of the voice and are found in the range from 300 to 1500 hertz Consonants occur between the ranges of 2000 and 4000 hertz.Now when is comes to intelligibility, those fundamental frequencies, down there between 100 and 250 HZ make some big waves but add almost nothing in the intelligibility.

Intelligibility lies in the mid ranges between 1000 and 7000 hertz. These frequencies contain little power but are most important for speech clarity. Rolling off some low frequencies  can help control a boomy vocal and by accentuating the range from 1 to 5KHz,  clarity can be improved.